Barrier valves or formation isolation valves or safety valves typically are used to keep one zone in a tubular string isolated from another. As a result such valves may see large differential pressure in the closed position. Equalizing valves have been developed to equalize pressure across a closed valve prior to trying to use the operating mechanism on the valve member. The reason for equalizing is that there is far less resistance on the operating mechanism for the valve when the pressure is equalized. Trying to operate the valve mechanism with a large differential pressure across it can distort or break the operating assembly making the valve immovable from the closed position or stuck part way open. Situations like this involve expensive fishing or milling operations to remove the inoperative valve.
Some designs use spring loaded poppets in flapper type safety valves such that when the flow tube descends it strikes the poppet first and opens a bypass passage around the flapper before the flow tube moves further and contacts the flapper to rotate it 90 degrees to the fully open position. Such designs are shown in U.S. Pat. No. 4,415,036 and U.S. Pat. No. 4,478,286. Other relevant prior art can be seen in U.S. Pat. No. 4,289,165; U.S. Pat. No. 8,534,361; US 20110088906 and U.S. Pat. No. 8,534,317. More recently and more relevant is U.S. Pat. No. 9,062,519 which is a design that is improved by the present invention. In this design there is a single control line 45 that pushes against a piston area 48 and against a return spring 50 to offset the hydrostatic pressure in the control line 45. In this design seal 18 has very low differential pressure that subjects it to leakage. Additionally, the size of the spring needed for deep setting depths makes the size of the device impractical for deployment in such depths. The present invention makes the equalizer valve insensitive to setting depth so that a specific spring does not need to be installed depending on the setting depth. Instead a pressure balanced two control line system is applied so that an external bias on an operating piston shaft is not needed although it could optionally be added.
In another aspect of the present invention, there are three seals that interact with the two line control system and a fourth seal that exerts a net closure force on the mandrel. That fourth seal is exposed to an atmospheric, or low, pressure on one side and upstream pressure on the opposite side ensuring optimum seal performance due to large differential pressure. Optionally, a spring can be used to overcome the friction of the seals in the system. These and other aspects of the present invention will be more readily apparent to those skilled in the art from a review of the detailed description of the preferred embodiment along with the associated drawing while recognizing that the full scope of the invention can be determined from the appended claims.